Titanium nitride is a titanium compound formed by reacting titanium metal with nitrogen. Zirconium nitride is formed by reacting zirconium metal with nitrogen. A coating of titanium nitride or zirconium nitride may be formed directly upon the surface of an object by physical vapor or chemical vapor deposition processes in a partially evacuated atmosphere. Physical vapor deposition embodies a number of deposition processes including reactive sputtering, D. C. sputtering, ion plating and arc evaporation deposition processes. The physical vapor deposition arc process uses a high current density electric arc to deposit a coating upon a substrate through the evaporation of source material. Chemical vapor deposition processes occur at relatively high temperature and are limited to substrate materials with mechanical and physical properties that are not adversely affected by process temperatures. Physical vapor deposition processes occur at much lower temperatures and can form a coating over a wide range of substrate materials such as carbide cermets, stainless steels, tool steels, superalloys and titanium alloys.
Titanium nitride has been successfully used as a protective coating for cutting and forming tools in the manufacturing industries and is presently recognized for its relatively high wear resistant and low frictional wear properties.
The titanium nitride crystal (TiN) and the zirconium nitride crystal (ZrN) have a NaCl type structure, consisting of two interpenetrating face-centered cubic lattices. X-ray diffraction techniques are commonly used to identify crystal structure and to determine crystallographic orientation and interplanar spacing. The intensity of x-ray diffraction from different planes of the crystal lattice is a measure of the crystallographic orientation. The interplanar spacing is a measure of the compressive stress level of the material. It is well recognized that titanium nitride and zirconium nitride, produced with physical vapor deposition and chemical vapor deposition processes, exhibit a preferred crystallographic orientation in the (111), (100) or (110) diffraction plane. Accordingly, the x-ray diffraction intensities for TiN and ZrN are commonly measured from these planes.
Although it is known that the x-ray diffraction intensity from polycrystalline titanium nitride and polycrystalline zirconium nitride may vary with the parameters of the physical vapor deposition arc process such as, for example, the configuration of the Ti or Zr cathode, chamber pressure, bias voltage, arc current, substrate standoff and temperature and the flow rate of gas through the chamber, only limited studies have been made relating to the effect on the physical properties of polycrystalline TiN or ZrN of variations in x-ray diffraction intensity. Prior art measurements have been made on flank wear of polycrystalline TiN. These measurements indicate that no significant improvement in the flank wear for polycrystalline TiN is realized when the x-ray diffraction intensity ratio I (111)I(200) is greater than 6. It has been observed in accordance with the present invention however that the erosion resistance of polycrystalline TiN and polycrystalline ZrN continue to increase as the x-ray diffraction intensity ratio I(111)/I(200) increases. Accordingly, the erosion resistance of polycrystalline TiN and polycrystalline ZrN can be controlled by controlling the x-ray diffraction intensity ratio I(111)/I(200).
It has been further discovered in accordance with the present invention that there is a minimum x-ray diffraction intensity ratio of I (111l)/I(200) for titanium nitride, measured as a ratio between the diffraction intensity from the (111) plane relative to the diffraction intensity from the (200) plane, necessary to provide a sound dense microstructure with superior erosion resistant characteristics. At an I (111)/I(200) x-ray diffraction intensity ratio of at least about 75 the TiN coating exhibits erosion resistance of a TiN coating produced by any of the prior art physical or chemical vapor deposition processes. At an I(111)/I(200) ratio of at least about 15, the ZrN coating exhibits erosion resistance which is substantially greater than the erosion resistance of a ZrN coating produced by any of the prior art physical or chemical vapor deposition processes. Moreover, the I (111)/I (200) x-ray diffraction intensity ratios of the titanium nitride crystal and the zirconium nitride crystal can be varied in accordance with the practice of the present invention to tailor the microstructure and erosion resistance of the coating for a particular application.
It has been observed also in accordance with this invention that the high angle (e.g. 90.degree.) impact erosion resistance of polycrystalline TiN and ZrN coatings and character of the erosion mechanism are a function of the residual coating stress as determined by the interplanar spacing, d.sub.111, of the (111) diffraction planes. Accordingly, the high angle impact erosion resistance and mechanism of the polycrystalline TiN or ZrN can be controlled by controlling the interplanar spacing of the (111) planes. It has been further discovered pursuant to this invention that there is a maximum interplanar spacing, d.sub.111, for each of the polycrystalline TiN and ZrN coatings, below which uniformly eroded surfaces and lower erosion rates have been observed to result from high angle (90.degree.) impact erosion and above which relatively large erosion pits due to intracoating spalling and relatively higher erosion rates have been observed in the eroded surfaces of the coatings when subjected to high angle (90.degree.) impact erosion. At or below an interplanar spacing, d.sub.111, of about 2.460 Angstroms for TiN coatings, and at or below an interplanar spacing, d.sub.111, of about 2.660 Angstroms for ZrN coatings, uniformly eroded surfaces and relatively lower erosion rates have been observed after being subjected to high angle (90.degree.) impact erosion whereas above these approximate values intracoating spalling and relatively higher rates of erosion have been experienced.